Institute of Electrical and Electronics Engineers (IEEE) 802.15.4e is an enhanced media access control (MAC) layer protocol of the IEEE 802.15.4 standard designed for low power and low data rate networks. The IEEE 802.15.4e architecture is defined in terms of a number of blocks in order to simplify the standard. These blocks are called layers. Each layer is responsible for one part of the standard and offers services to the higher layers. The interfaces between the layers serve to define the logical links that are described in the standard. A low-rate (LR)-wireless personal area network (WPAN) device comprises at least one PHY (physical layer), which contains the radio frequency (RF) transceiver (or radio) along with its low-level control mechanism, and a MAC sublayer that provides access to the physical channel for all types of transfers. The radio device can either transmit or receive at any given time, but cannot simultaneously perform both transmitting and receiving.
IEEE 802.15.4e is suitable for sensor devices with resource constraints; e.g., low power consumption, low computation capabilities, and low memory. As sensors and actuators that are interconnect by a PAN in home and office environments become more common, limiting power dissipation for each device is important. Some radio devices may operate on a battery, in which case frequent battery changes or recharges are undesirable. Some other radio devices may operate on a limited amount of power that is generated by the device itself such as using conversion from solar or other light sources, energy scavenging from motion or thermal effects, or collection of energy from ambient electromagnetic fields.
Channel hopping wireless transmission system protocols typically have a retransmission mechanism to retransmit lost frames. When channel hopping is used, subsequent transmissions can use a different channel (frequency band) in the channel hopping sequence. This helps avoid channel interference that may have existed in the previous channel used causing frame loss so that channel hopping can improve network capacity. Channel hopping achieves increased network throughput by promoting simultaneous data transfer over multiple channels between different pairs of radio devices, or to achieve reliability in tough channel conditions by exploiting channel diversity.
Channel hopping can be achieved through many different known methods, with the most common methods used being either a synchronous method called Time Slotted Channel Hopping (TSCH) or an asynchronous method called un-slotted channel hopping as defined in the IEEE 802.15.4e standard. Many standards also exist that use a channel hopping MAC to define MAC protocols for different applications. Standards also exist that use a channel hopping MAC to define MAC protocols for different applications. For example the Wi-SUN™ Alliance has published a Field Area Network (FAN) specification that specifies how to use asynchronous channel hopping for smart grid applications (Technical Profile Specification Field Area Network, Wi-SUN Alliance 2014, hereafter the “Wi-SUN FAN”).
In asynchronous channel hopping (which does not require any synchronization for channel hopping) MACs, such as the one defined in the Wi-SUN FAN, each radio device maintains its broadcast schedule as well as a unicast schedule. The radio device will transmit its broadcast data during its broadcast schedule and its neighboring radio devices who are already tracking this radio device are expected to be listening in its channel during the broadcast slot. During the broadcast interval, the radio devices follow their own receiver directed unicast channel hopping schedules. Being asynchronous channel hopping, the unicast channel hopping slots need not be synchronized to each other.
The unicast schedules are receiver directed in the sense that a radio device transmits the frame in the receiver radio device's channel using carrier sense multiple access with collision avoidance (CSMA/CA). If the frame's data transmission goes beyond the slot period then the transmitting radio device continues the data transmission into the adjacent slots of the receiver radio device as well. For example, responsive to a data request received from a radio device B, a radio device A may transmit a frame in radio device B's Rx channel and the data transfer from radio device A may extend into the adjacent time slot. The channel hopping information is exchanged between the respective radio devices A and B through the transfer of special information elements as disclosed in the Wi-SUN FAN.
TSCH uses dedicated/shared time slots for radio devices and thus the devices do not need to have a complex channel access procedure. Asynchronous channel hopping on the other hand needs a channel access mechanism to transmit frames because the radio devices can transmit frames at any time. However, known asynchronous access mechanisms do not take into account the presence of interference that may be present at certain random times on some channels.